This invention relates to an optical element which is usable in an optical system for a camera, a telescope, a microscope, or a semiconductor manufacturing apparatus, for example.
FIG. 16 is a schematic view for explaining gravity (weight) deformation of a certain substrate 1. In FIG. 16, the substrate 1 is made of quartz having a diameter 150 mm and a thickness t. It is supported by supporting means 2, at a distance of radius a. Where the thickness t of the substrate 1 is small, there occurs a deformation W of the substrate 1 by gravity (weight), at a central portion thereof.
Here, if the thickness t of the substrate 1 is t=0.8 mm, the radius a of the supporting means 2 is a=70 mm, the Young""s modulus E of quartz is E=7.31xc3x97104N/mm2, the Poisson""s ratio "ugr" is "ugr"=0.17, the density xcfx81 is xcfx81=2.22xc3x9710xe2x88x926Kg/mm3, the amount of deformation W can be determined in accordance with equation (1) below, and the deformation amount at the central portion of the substrate 1 is W=7.2 microns.
W=3(1xe2x88x92"ugr"2)9.81xcfx81ta2{(5+"ugr")a2/(1+"ugr")}/16Et3xe2x80x83xe2x80x83(1) 
FIG. 17 is a schematic view of a binary optics element 11 which is a diffractive optical element having a very small thickness. If the binary optics element 11 is supported by supporting means 2 of a radius 70 mm, then, as shown in FIG. 18, there will occur deformation in the thin binary optics element 11 due to the gravity, like the example of FIG. 16.
It is an object of the present invention to provide an optical element by which deformation such as described above can be avoided or reduced to a level that can be disregarded.
The amount of deformation of an optical element due to the gravity (weight) or the like can be predicted, by calculation, in accordance with the shape or material of the optical element or with the holding method for the optical element, for example. Also, any optical measurement device may be used to practically measure the amount of deformation. In accordance with the present invention, an optical element may be provided with a film formed on the surface thereof and having a stress value effective to produce a deformation amount cancelling the deformation amount as described above.
More specifically, in accordance with an aspect of the present invention, there is provided an optical element, characterized in that said optical element is provided with a film having a stress for substantially suppressing deformation of said optical element.
The deformation of said optical element may be gravity deformation. The film may comprise an anti-reflection film. The center of deformation of said optical may be substantially registered with an optical center of said optical element. The optical center of said optical element is substantially registered with a center of a substrate.
In accordance with another aspect of the present invention, there is provided an optical instrument having an optical element such as described above.
In accordance with a further aspect of the present invention, there is provided an exposure apparatus having an optical instrument as described above.
In accordance a yet further aspect of the present invention, there is provided a device manufacturing method, comprising the steps of: exposing a wafer to a device pattern by use of an exposure apparatus as described above, and developing the exposed wafer.